[isFmt, fmtContents, labelFmt] = _g_parse_oct_fmt (FMT, plotn, wantLabel)
Take a string FMT, e.g. "-5;Foobar;", in octave's plot (x,y,FMT) format and
tries to translate it into fmtContents, a gnuplot string like
"with lines linetype 5 title 'Foobar'".
isFmt is 1 if FMT makes sense in octave's plot syntax, 0 otherwise.
If the wantLabel is true, or if the "format" part of FMT ("-5", in the
example above) contains a "#", then labelFmt will hold a gnuplot string that
plots a label with each data point.
